It is well known that well streams containing a mixture of fluids such as crude oil, condensate, formation water and gas during transportation may react and form solid hydrates resulting in i.e. blocked pipelines. To avoid and or limit the formation of hydrates inhibitors are added to the well stream prior to transportation. One often applied hydrate inhibitor is mono-ethylene glycol (MEG); other applicable hydrate inhibitors include glycol compounds with other substituents, as well as kinetic hydrate inhibitors or a combination thereof. The term kinetic refers to the effect of the inhibitor lowering the reaction rate of the hydrate forming reactions.
Hydrate inhibitors such as MEG are valuable chemicals and the recycling thereof provides reduced costs. However equally important are the environmental consequences, as waste streams containing not inconsiderable amounts of MEG or similar inhibitors can not be released to the environment.
A number of different steps and methods for separating MEG for reuse are known in the art.
After a first separation of hydrocarbons the process fluid normally comprises a hydrocarbon rest, water, corrosion products, MEG and dissolved inorganic salts. Normally the amount of inorganic salts is considerable and the salts may precipitate during the MEG extraction process which again leads to increased viscosity, sludge formation or plugging. Depending on the formation the composition of the inorganic salts will vary. It is well known to remove salts from solutions by precipitation of solids by increasing the activity or concentration; however the combination of alkali and earth alkali ions, and halogen, carbonate and bicarbonate ions results in the precipitation of relatively small salt particles that are not easily separated from the solution. A further obstacle for the regeneration process is degradation of the inhibitor at high temperatures which limits the possibility to use heating to obtain separation.
Monoethylene glycol (MEG) is added to gas transport pipelines to avoid gas hydrate formation in long tie backs. Gas, MEG, H2O are separated downstream of the pipeline at the receiving facilities. MEG at these facilities contains formation water, condensed water and salts. This MEG is regenerated in a MEG reclamation plant by removing salts and water, ensuring that the MEG is reusable. Salts arriving at the MEG Regeneration system are in the form of dissociated ions and come primarily from the well's formation water, pipeline corrosion and chemical injection (pH stabiliser). Some of these salts are also from organic acids (formic, acetic, propanoic, butanoic etc.) in the formation water and gas; these are sometimes also called volatile fatty acids (VFA). Sodium and potassium salts of carboxylic acids (acetate, formate, propionate, butrate, etc.) are highly soluble in MEG under reclaimer conditions. Solubility of these salts increases with increase in temperature and hence it is not possible to precipitate the salts of carboxylic acids in the reclaimer. Since these salts are not precipitated in the reclaimer, they are not removed from the MEG system and they keep on accumulating in the reclaimer as dissolved salts. Beyond a certain accumulation they increase the viscosity of the reclaimer slurry and thereby influence the performance of the pumps and heat exchangers. At the moment there is no proven technology to precipitate/remove the salts of carboxylic acids from the MEG reclaimer.